Trailer trucks form one of the major components of the goods transportation system in modern commerce. In the most common rig, a motorized tractor unit pulls a freight carrying semi-trailer unit. The latter rides on a set of wheels, usually two or more to a side, supporting the rear end. The front end rests on a pivot on the tractor unit. Behind the pivot, the semi-trailer has a small dolly, usually wheeled, which normally is too short to reach the ground or is folded up under the bottom of the semi-trailer. When the tractor is being detached the dolly is unfolded or lowered by ratchets so that the front end of the semi-trailer is supported and the tractor can disengage.
Such semi-trailers are customarily loaded and unloaded in truck yards in which a series of parking lanes are perpendicular to a loading platform or dock the same height as the floor of the semi-trailer. When the latter is backed against the loading platform goods can be mechanically loaded or unloaded by such machines as fork lifts which run right into the semi-trailer. For this reason it is necessary that the semi-trailer be properly immobilized, during the operation, by chocking the wheels.
When a semi-trailer is backed up to a loading platform, the driver, as of now, must perform a number of operations. The tractor must be disconnected (requiring the dolly to be lowered) and then driven away. As a separate operation, the wheels of the trailer must be blocked by large wooden chocks manually pounded into the angle between the tires and the pavement. When a trailer is to be hauled away these steps must be reversed. Chocking the wheels requires a measurable amount of the driver's valuable time and effort. Further, the chocks are usually made of wood and are expensive. They wear out and must be replaced several times a year. They also serve as a standing temptation for pilferage, since drivers are always in need of such chocks for emergency use. There is thus a strong economic need for a device which will automatically chock the wheels of a parked semi-trailer.
Some of the same or allied problems arise with recreational camper and boat trailers. In particular, the smaller trailers do not have good brakes and can easily run away when parked. Also, dealers and the like have a need for a method of effectively immobilizing such vehicles on their lots.